Additional Study Information: The purpose of this clinical trial to find out if the investigational medication pembrolizumab in combination with enzalutamide + Androgen Deprivation Therapy (ADT) is safe and works to slow down or stop the growth of metastatic prostate cancer compared to enzalutamide + ADT alone. Pembrolizumab is a type of immunotherapy, which may help the bodys immune system attack cancer cells. KEYNOTE-991 will be enrolling patients diagnosed with metastatic prostate cancer at initial diagnosis and patients who develop metastatic disease following primary therapy for locally advanced prostate cancer.